Electrochemical-based purification of electrolyte solutions, and related systems and methods

ABSTRACT

Methods and systems for removing impurities from electrolyte solutions having three or more valence states. In some embodiments, a method includes electrochemically reducing an electrolyte solution to lower its valence state to a level that causes impurities to precipitate out of the electrolyte solution and then filtering the precipitate(s) out of the electrolyte solution. In embodiments in which the electrolyte solution is desired to be at a valence state higher than the precipitation valence state, a method of the disclosure includes oxidizing the purified electrolyte solution to the target valence.

RELATED APPLICATION DATA

This application claims the benefit of priority of U.S. ProvisionalPatent Application Ser. No. 62/405,576, filed on Oct. 7, 2016, andtitled “REDUCTION-BASED PURIFICATION OF ELECTROLYTE SOLUTIONS ANDRELATED SYSTEMS AND METHODS”, which is incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

The present invention is related to purification and production ofelectrolyte solutions. More particularly, the present invention isdirected to electrochemical-based purification of electrolyte solutions,and related systems and methods.

BACKGROUND

Electrolytes for batteries and other uses generally need to be free ofimpurities that are detrimental to those uses. For example, in thecontext of a redox flow battery, each electrolyte used needs to be freeof impurities that foul components of the battery. In a particularexample, a vanadium redox flow battery (VRFB) is a system that convertselectrical energy into chemical energy and then releases that chemicalenergy as electricity when there is demand. This type of battery isoften paired with a solar and/or wind farm to help smooth out the powerproduction intermittency associated with these renewable energy sources.

A VRFB comprises an electrochemical cell that performs the conversionbetween chemical and electrical energy. The electrochemical cellincludes a negative electrode, an electrolyte separator (often a protonexchange membrane), and a negative electrode. Two separate vanadiumsolutions are stored in individual tanks—one tank contains a negativeelectrolyte solution that is fed to the negative electrode, and theother tank contains a positive electrolyte solution that is fed to thepositive electrode. During normal operation, the negative electrolytesolution contains vanadium (II) and (III) ions, and the positiveelectrolyte solution contains vanadium (IV) and (V) ions. During charge,vanadium (III) ions are reduced to vanadium (II) ions in the negativeelectrolyte solution at the negative electrode, and vanadium (IV) ionsare oxidized to vanadium (V) ions in the positive electrolyte solutionat the positive electrode; the opposite happens during discharge.

When commissioning a new VRFB, a balanced electrolyte solution ofaverage valence of roughly 3.5, i.e., an electrolyte solution of equalconcentration of vanadium (III) and vanadium (IV) ions, is transferredinto both the negative and positive electrolyte tanks. The battery isslowly charged until the negative and positive electrolyte solutions areat the desired ratio of vanadium (II)/(III) and vanadium (IV)/(V) ions,respectively. After this initial charge, some impurities present in thenegative electrolyte solution will typically precipitate out as a solidmetal phase precipitate. These impurities include, but are not limitedto, As and Ge. These precipitates are detrimental to the electrochemicalcell, because they clog the negative electrode and negatively impactbattery performance.

The majority of conventional methods for making a vanadium-basedelectrolyte solution involve one of two methods:

-   -   Method 1: Mixing V₂O₃ and V₂O₅ in a 3:1 molar ratio in excess        acid to produce a solution of a 3.5 average valence.    -   Method 2: Using a VRFB electrochemical cell in which the        negative electrode is used to reduce a vanadium-based        electrolyte solution to a 3.5 valence and the positive electrode        is oxidizing a vanadium-based electrolyte solution that is        periodically or continually reduced using a chemical reducing        agent. This type of approach is needed because most organic        reducing agents are only able to reduce vanadium (V) ions to        vanadium (IV) ions (i.e. most organic reductants can't        chemically reduce vanadium (IV) ions to a lower valence).

Neither of these conventional methods adequately remove impurities thatoften foul the electrochemical cell of a VRFB. Rather, both methodsrequire that the vanadium feedstocks have low impurity content of selectimpurities for proper function and inhibiting fouling in a VRFB system.Consequently, many purification methods focus on the purification of thevanadium feedstock. While many of these methods are effective, theyconsume many chemicals and the final product commands a high costpremium.

SUMMARY OF THE DISCLOSURE

In one implementation, the present disclosure is directed to a method ofmaking a purified electrolyte solution. The method includes providing anelectrolyte solution having a precipitation valence and containing atleast one impurity that precipitates out of the electrolyte solutionwhen the valence of the electrolyte solution is at or below theprecipitation valence; reducing the electrolyte solution to a valencebelow the precipitation valence so as to cause the at least one impurityto precipitate out of the electrolyte solution as a precipitate; andmechanically separating the precipitate out of the electrolyte solutionso as to make the purified electrolyte solution.

In another implementation, the present disclosure is directed to amethod of commissioning a redox flow battery having a positive side anda negative side. The method includes providing an electrolyte solutionhaving an initial valence and at least four available oxidation statescomprising a set of higher oxidation states and a set of lower oxidationstates; reducing the electrolyte solution to a valence below the initialvalence to make a reduced electrolyte solution having a valence withinthe set of lower oxidation states; oxidizing a first portion of thereduced electrolyte solution to make a positive-side electrolytesolution having a valence within the set of higher oxidation states;providing, for the commissioning, the positive-side electrolyte solutionfor the positive side of the redox flow battery; and providing, for thenegative side of the redox flow battery for the commissioning, a secondportion of the reduced electrolyte solution as a negative-sideelectrolyte solution having a valence within the set of lower oxidationstates.

In yet another implementation, the present disclosure is directed to asystem for making a purified electrolyte solution from an electrolytesolution containing at least one impurity that precipitates out of theelectrolyte solution at or below a precipitation valence. The systemincludes a reduction system that includes an electrochemical reductioncell designed and configured to electrochemically reduce the electrolytesolution based on a reductant; a recirculation loop designed andconfigured to recirculate at least a portion of the electrolyte solutionto the electrochemical reduction cell; a reduction-process controlsystem in operative communication with the reduction system so as tocontrol the valence of the electrolyte solution in the reduction systemto a desired value at or below the precipitation value so as to causethe at least one impurity to precipitate out of the electrolyte solutionas a precipitate; and a solid/liquid mechanical separation systemdesigned and configured to remove at least a portion of the precipitateout of the electrolyte solution to make the purified electrolytesolution.

In still yet another implementation, the present disclosure is directedto a system for making a valence-adjusted electrolyte solution from anelectrolyte solution having at least four oxidation states. The systemincludes a reduction system that includes an electrochemical reductioncell designed and configured to electrochemically reduce the electrolytesolution based on a reductant; a recirculation loop designed andconfigured to recirculate at least a portion of the electrolyte solutionto the electrochemical reduction cell; a reduction-process controlsystem in operative communication with the reduction system so as tocontrol the valence of the electrolyte solution in the reduction systemto a desired value; and an output designed, configured, and located tooutput the electrolyte solution as previously reduced electrolytesolution at about the desired value; and an oxidation system thatincludes an electrochemical oxidation cell designed and configured toelectrochemically oxidize the previously reduced electrolyte solutionbased on an oxidant; a recirculation loop designed and configured torecirculate at least a portion of the previously reduced electrolytesolution to the electrochemical oxidation cell; an oxidation-processcontrol system in operative communication with the oxidation system soas to control the valence of the previously reduced electrolyte solutionin the oxidation system to a desired final value; and an outputdesigned, configured, and located to output the previously reducedelectrolyte solution at about the desired final value.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, the drawings show aspectsof one or more embodiments of the invention. However, it should beunderstood that the present invention is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 is a graph of valence versus time for an example electrolyteformation/purification and valence adjustment process in which theexample electrolyte solution has four oxidation states ranging from +2to +5;

FIG. 2 is a schematic diagram of an example continuous electrolyteformation/purification and valence-adjustment process;

FIG. 3A is a diagram illustrating an example construction of thereduction cell of FIG. 2;

FIG. 3B is a diagram illustrating an example operation of the reductioncell having the construction of FIG. 3A, showing that a vanadiumelectrolyte is reduced and hydrogen gas is oxidized;

FIG. 4A is a diagram illustrating an example construction of theoxidation cell of FIG. 2;

FIG. 4B is a diagram illustrating an example operation of an oxidationcell having the construction of FIG. 4A, showing that a vanadiumelectrolyte is oxidized while protons are reduced to hydrogen gas;

FIG. 5 is a schematic diagram of the reduction/purification sub-processused in experimental lab-scale testing;

FIG. 6A is a graph of cell operating and open-circuit voltages versustime (hours) in the reduction cell of FIG. 5 for Batch 1 containing 3liters of impure electrolyte containing high levels of As (5 ppm) and Ge(0.2 ppm) and having a starting valence of 3.5;

FIG. 6B is a graph of pressure versus time (hours) for Batch 1 forpressure just upstream of the filter prior to the reduction cell of FIG.5 and the pressure at the inlet to the reduction cell;

FIG. 7A is a graph of cell operating and open-circuit voltages versustime (hours) of the reduction cell of FIG. 5 for Batch 2 containing 3liters of impure electrolyte containing high levels of As (5 ppm) and Ge(0.2 ppm) and having a starting valence of 3.5;

FIG. 7B is a graph of pressure versus time (hours) for Batch 2 forpressure just upstream of the filter prior to the reduction cell of FIG.5 and the pressure at the inlet to the reduction cell;

FIG. 8 is a schematic diagram of the oxidation sub-process used in theexperimental lab-scale testing;

FIG. 9A is a graph of voltage versus time (hours) for the oxidation ofthe Batch 1 electrolyte in the oxidation sub-process of FIG. 8;

FIG. 9B is a graph of current density versus time (hours) in theoxidation cell of FIG. 8;

FIG. 10 is a graph of negative electrode pressure versus time (hours)for both of the as-made electrolyte of Batch 1 and the electrolyte ofBatch 1 after reduction/purification using the experimental process;

FIG. 11A is a graph of cell and reference cell voltages versus time(hours) for Batch 1 after reduction/purification using the experimentalprocess;

FIG. 11B is a graph illustrating the Coulombic efficiency, energyefficiency, and vanadium utilization over time (hours) for the purifiedBatch 1;

FIG. 11C is a graph of pressure versus time (hours) showingnegative-side and positive-side electrolyte pressures;

FIG. 12A is a diagram illustrating the reduction sub-process for apurification system that uses an electrochemical cell that accepts VRBelectrolyte on both positive and negative electrodes;

FIG. 12B is a diagram illustrating the oxidation sub-process for apurification system that uses an electrochemical cell that accepts VRBelectrolyte on both positive and negative electrodes;

FIG. 13 is a diagram illustrating an electrochemical cell that can beused in each of the reduction and oxidation sub-processes illustrated,respectively, in FIGS. 12A and 12B; and

FIG. 14 is a diagram illustrating the reactions present in theelectrochemical cell of FIG. 13.

DETAILED DESCRIPTION

In some aspects, the present disclosure is directed to methods andsystems for removing impurities from an electrolyte solution using achemical reduction and filtration process. A particularly usefulembodiment is described below in the context of a vanadium redox flowbattery (VRFB) in which one or more impurities in the vanadium-basedelectrolyte solution that would in the normal course clog theelectrochemical cell of the VRFB during normal use are removed. Anoxidation process, such as an electrochemical oxidation process, can beused after filtration to adjust the valence of the reduced electrolytesolution to a desired value, often an electrolyte 3.5 valence.

In other aspects, the present disclosure is directed to methods andsystems directed to and relating to commissioning a redox flow battery(RFB). In a particular embodiment, an electrolyte solution is firstreduced to a relatively low oxidation valence and then oxidized to makenegative and positive electrolyte solutions having differing valences.The negative and positive electrolyte solutions are installed into thenegative and positive sides of an RFB, respectively, thereby eliminatingthe need for the typical battery-charging process that accompaniesconventional battery commissioning. These and other aspects of thepresent disclosure are described and exemplified below in detail.

Referring now to the drawings, FIG. 1 illustrates an overall process ofmaking an electrolyte solution having a desired valence byreduction-based purification followed by filtration and oxidation-basedvalence adjustment. To understand the purification process, i.e., theelectrochemical reduction and filtering shown in FIG. 1, it must beunderstood that the purification at issue involves the removal of solidprecipitates that form from impurities in the electrolyte solution at arelatively low vanadium valence. In some electrolyte solutions, certainimpurities are in aqueous states when the valence of the electrolytesolution is relatively high but are transformed to their solid formswhen the valence of the electrolyte solution is sufficiently low.Depending on the use of the electrolyte solution, the solids formed whenthe electrolyte solution's valence is relatively low can be detrimentalto that use. For example, in the context of an RFB, impurity solids onthe negative side of the RFB's electrochemical cell can clog thenegative electrode, causing the electrical performance of the RFB todegrade as the negative electrode becomes increasingly clogged by thesolids over time.

Using a VRFB as an example, in order to manufacture electrolytesolutions for a VRFB cost effectively, an inexpensive source of vanadiumoxides is required. Generally, low price vanadium oxides have highlevels of impurities that end up in the electrolyte solutions and have alarge negative impact on VRFB performance. As noted above, the mostdetrimental impurities are the ones that are aqueous in the as-receivedelectrolyte solution, but will precipitate out as solids that clog thenegative electrode after the system's initial charge. In other words,the most detrimental impurities are soluble/aqueous in an electrolytesolution of vanadium (III)/(IV) ions but are insoluble (i.e. form solidprecipitates) in a solution of vanadium (II)/(III) ions. Thepurification process illustrated in FIG. 1 can readily be used to filterthe precipitatable impurities out of the electrolyte solution prior tousing the solution in a VRFB.

Referring particularly to FIG. 1, this figure illustrates purificationand valence-adjustment performed in accordance with the presentdisclosure in the context of an example electrolyte solution in whichthe electrolyte has four possible oxidation states, here, +2, +3, +4,and +5. As seen in zone 100 of FIG. 1, the purification process involvesfirst reducing the electrolyte solution to a valence that is lower thanthe valence at which the target impurity(ies) precipitate(s) out of theelectrolyte solution. The starting valence in zone 100 can be anywherefrom 3.5 to 5.0, depending on the vanadium oxide feedstock used; in FIG.1 we assume V₂O₅ is the sole feedstock and the resulting startingvalence is 5.0. In the example illustrated, the electrolyte solution isreduced approximately to its lowest oxidation state (here, an oxidationstate of (II) (2.0)). For a vanadium-based electrolyte solution, forexample, target impurities precipitate out of the solution at a valenceat or below a valance of 3.0. Consequently, in some embodiments, thereduction need only take the valence of the electrolyte solution belowabout 3.0, such as 2.9 to 2.0. However, a lower valence can be desirableto accelerate the reduction process. The reduction can be performedusing any suitable process, such as electrochemical reduction using anelectrochemical cell. In one example, a hybrid electrochemical cell maybe used, wherein one half-cell utilizes a VRB electrolyte, and the otherhalf-cell uses a non-VRB reductant. In another example, theelectrochemical cell may be configured to accept a VRB electrolyte onboth halves of the cell.

Once the impurities precipitate out of the electrolyte solution, in zone104 of FIG. 1 solid precipitates are removed from the solution.Precipitate removal can be performed using any suitable removal means,such as via one or more porous filters and/or one or more cyclones,among other things. Depending on the use of the electrolyte solution,only the precipitates of a certain size may need to be removed, suchthat the removal equipment can be designed accordingly. For example, ina VRFB that includes a negative electrode having a particular pore size,precipitates of a certain size smaller than this pore size may not needto be filtered, since they can flow through the negative electrode andnot clog it.

At an optional step, illustrated in zone 108 of FIG. 1, the purifiedelectrolyte solution, i.e., the solution having the desired amount ofprecipitate removed, may be oxidized to a desired valence. As an exampleand using a vanadium electrolyte, the purified electrolyte solution maybe oxidized so that it has a mix of vanadium (III) and vanadium (IV)ions and a valence of about 3.5, which is the valence of conventionalvanadium-based electrolyte solution typically provided for commissioningVRFBs. This mix is represented by line 112 in FIG. 1.

As another example using vanadium as the electrolyte, and as shown inFIG. 1, purified electrolyte solution may be oxidized to create twoelectrolyte solutions of differing valences, such as one electrolytesolution of valence at or below 3.0, such as a valence of 2.5,containing vanadium (II) and vanadium (III) ions and another electrolytesolution of valence at or above 4.0, such as a 4.5 valence, containingvanadium (IV) and vanadium (V) ions. These two electrolyte solutions arerepresented, respectively, in FIG. 1 by lines 116 and 120. In thisexample, these two electrolyte solutions can be used, respectively, asthe negative and positive electrolyte solutions of a VRFB.

As mentioned above, when a VRFB is conventionally commissioned, anelectrolyte solution of 3.5 valence is provided to both the negative andpositive electrolyte tanks of the VRFB, and then the battery is chargedso that the negative-side electrolyte solution settles at a valence ofabout 2.5 and the positive-side electrolyte solution settles at avalence of about 4.5. However, when the method of FIG. 1 is used to makethe negative (such as a 2.5 valence) and positive (such as a 4.5valence) electrolyte solutions and these electrolyte solutions are addedto the VRFB, the commissioning charge step can be eliminated. It isnoted that if the feedstock electrolyte solution is sufficiently pure,i.e., is sufficiently devoid of precipitatable impurities, then theimpurity removal step can be eliminated in the process of making thenegative and positive electrolyte solutions. This embodiment may beparticularly useful when a high-oxidation-state material, such as V₂O₅is used as sole vanadium feedstock for making the electrolyte.

In some embodiments, any of the methods represented by FIG. 1 may beperformed at an electrolyte solution production facility such that theresulting electrolyte solution(s) is/are transported to the location ofuse, such as VRFB installation quite remote from the productionfacility. However, the cost of transporting electrolyte solutions can bequite high due to factors such as distance, corrosivity of theelectrolyte solutions, and the fact that a large percentage of theweight of the solutions is due to the solvents (e.g., acid and water) inthe solutions. Consequently, in other embodiments, any of the methodsrepresented by FIG. 1 may be performed at or in close proximity to thelocation of use. For example, components, such as reduction equipment,precipitate removal equipment, and oxidation equipment, of a system canbe containerized, palletized, or otherwise made readily transportable sothat the method(s) can be performed locally.

In some embodiments, electrochemical cells are used for reduction andoxidation, and those cells need to be energized with electricity todrive the reactions. In such embodiments that are used proximate to theuse locations of the fabricated electrolyte solutions and in which thesolutions are used for batteries for renewable energy sources, such aswind turbines and solar farms, the electricity needed for the reductionand/or oxidation processes can be provided by the renewable energysources.

Advantages of methods of the present disclosure over existing methodsinclude:

-   -   No prior method utilizes precipitation of impurities for        removal. Precipitated impurities are removed mechanically, for        example, via filtration and/or a hydrocyclone.    -   No prior method utilizes both an electrochemical oxidation and        reduction of the electrolyte for the formation and removal of        key impurities.    -   No prior method uses electrochemical cells for reduction and        oxidation of the VRB electrolyte. An example of a cell for        vanadium-based electrolyte reduction is a hydrogen/VRB cell,        where H₂ gas is oxidized (consumed) at the cathode and VRB        electrolyte is reduced at the anode. The oxidation cell is        similar: H₂ is produced on the anode, and VRB electrolyte is        oxidized on the cathode.    -   No other method allows the production of both separate anolyte        and catholyte solutions for a VRFB system using the same        electrochemical system.

FIG. 2 illustrates an example purification process 200 that uses twoelectrochemical sub-processes, namely, a reduction/purificationsub-process 200(1) for reducing and purifying the electrolyte solutionand an oxidation sub-process 200(2) for adjusting the valence of theelectrolyte solution to a desired valence. Process 200 of this exampleenables the use of vanadium oxides of a single oxidation state and ofreduced purity. The formation and purification of the electrolytesolution in example process 200 is performed in a continuous manner, andthe process is designed to operate at room temperature.

Example process 200 removes impurities that are the most detrimental toVRFB performance, namely, impurities that are aqueous in a mixedvanadium (III)/(IV) solution but that will form solid precipitates aftercharging to make vanadium (II)/(III) solution. As noted above, it isthese precipitates that can clog the negative electrode. Thepurification performed by process 200 relies on the discovery that themost detrimental impurities will precipitate out in a vanadium(II)/(III) solution and that these impurities can be removedmechanically, such as by filtration and/or cycloning, among otherthings. In this example, other impurities, such as K, Na, and Al, arenot removed in process 200, because they remain as aqueous ions in avanadium (II)/(III) solution; however, the impact of these aqueous ionson battery performance is negligible.

Electrochemical reduction and subsequent oxidation of the vanadium-basedelectrolyte are critical to its formation and purification. In thisexample, the reduction and oxidation of the electrolyte solution areperformed in two separate sub-processes, each using one or more of itsown electrochemical cells. It is noted that for the sake ofillustration, only a single electrochemical cell is shown for each ofthe two sub-processes 200(1) and 200(2), though each sub-process coulduse two or more electrochemical cells.

Reduction/purification sub-process 200(1), wherein electrochemicalreduction of the vanadium electrolyte occurs, is performed by a“reduction cell” 204, such as a hybrid electrochemical cell. Electricity(not illustrated) is provided to operate reduction cell 204, andvanadium-based electrolyte solution 208 is reduced on the cathode 204C,and a reductant 212 is oxidized on the anode 204A. In the illustratedexample, reductant 212 is H₂ gas, which provides for a relatively simpleand inexpensive system. However, other reductants, such as water, formicacid, ethylene glycol, among others, could be used for reductant 212.

FIG. 3A illustrate an example construction of a reduction cell 300 thatcan be used for reduction cell 204 of FIG. 2. Referring to FIG. 3A,example reduction cell 300 includes an electrolyte flow field 304, a gasflow field 308 (here, for H₂ gas), a proton-exchange membrane 312, acarbon-paper electrode 316, and a gas diffusion layer 320 coated with anH₂ oxidation reaction catalyst (not shown). Electrolyte solution 208(FIG. 2) is flowed into electrolyte flow field 304, while reductant 212,here, H₂ gas, is flowed into gas flow field 308. FIG. 3B illustrates theoperation example reduction cell 300, with block 324 representing theanode side of the reduction cell receiving H₂ reductant 212, block 328representing the cathode side of the reduction cell receivingelectrolyte solution 208, and block 332 representing proton-exchangemembrane 312 that allows hydrogen protons to pass from the cathode sideto the anode side, with the flow of electrons following suit. FIG. 3Balso illustrates the reduction of vanadium-based electrolyte solution208 and the oxidation of H₂ reductant 212. Those skilled in the art willreadily understand, the construction of reduction cell 300 illustratedin FIG. 3A is merely an example and that other constructions may be usedas desired.

Referring again to FIG. 2, reduction/purification sub-process 200(1)performs two functions: 1) it creates vanadium (II) ions that aid in thechemical dissolution and reduction of an electrolyte 216, here V₂O₅powder, and 2) it precipitates out the most deleterious impurities 220and enables their removal via mechanical means (e.g., filtration and/orcycloning). In the example shown in FIG. 2, system 200 includes one ormore suitable filters, here a coarse filter 224 and a pair of finefilters 228(1) and 228(2). The result is a purified electrolyte solution208P that is purified to the extent that some or all of the precipitatedimpurities 220 have been removed, again, here, by filters 224, 228(1),and 228(2).

Reduction/purification process 200(1) may be controlled by anappropriate controller 264 that controls the process. Controller 264 mayinclude any suitable hardware, such as a programmable logic controller,general purpose computer, application-specific integrated circuit, orany other hardware device(s) capable of executing a suitable controlalgorithm. Many types of hardware suitable for controller 264 are wellknown in the art. In one example, controller 264 is configured, viasoftware or otherwise, to control the valence of the electrolytesolution flowing out of reduction cell 204, here electrolyte solution208(2.0), by controlling the flow of impure electrolyte solution 208from tank 252 into the reduction cell. In this example, inputs to thecontrol algorithm include user settings, such as the electrical currentwithin reduction cell 204, and a valence measurement of the electrolytein tank 252 by a suitable valence sensor (not shown). For example, witha fixed cell current, controller 264 determines whether or not themeasured valence is below a target value (e.g., the precipitationvalence) and outputs one or more control signals that control one ormore flow-control devices (not shown) that in turn control the flow ofimpure electrolyte solution 208 into reduction tank 252. Suchflow-control devices may be, for example, one or more pumps, one or morevalves, or one or more devices that changes the flow of impure feedstockinto tank 252, or any combination of these, among others. In otherembodiments, controller 264 can be configured to control both cellcurrent and flow of impure electrolyte solution 208 so as to control thevalence of the electrolyte solution within tank 252. Those skilled inthe art will readily understand how to create a suitable algorithm forthe control scheme selected for controller 264 based on this disclosureand for the type of hardware used.

Oxidation sub-process 200(2) in this example uses electrochemicaloxidation to oxidize the purified electrolyte solution to bring it up toits desired valence state (such as near a 3.5 valence, near a 2.5valence, and/or near a 4.5 valence, depending on the desired use). Inthis example, a hybrid electrochemical cell, referred to herein as an“oxidation cell” 232, is used to drive the oxidation in response toelectricity (not illustrated) provided to the cell. Purifiedvanadium-based electrolyte solution 208P is oxidized on the anode 232A,and an oxidant 236 is reduced on the cathode 232C. In one example,protons are reduced to form H₂ gas on cathode 232C. This is particularlyconvenient when H₂ gas is used as reductant 212 in the reduction cell204 as mentioned above. After oxidation, the valence-adjustedelectrolyte solution(s) may be optionally transferred to a VRFB, asillustrated at box 240.

FIG. 4A illustrate an example construction of a reduction cell 400 thatcan be used for oxidation cell 232 of FIG. 2. Referring to FIG. 4A,example oxidation cell 400 includes an electrolyte flow field 404, anoxidant flow field 408 (here, for H₂ gas), a proton-exchange membrane412, an oxidation-side carbon-paper electrode 416, a reduction-sidecarbon-paper electrode 420, and a gas diffusion layer 424 coated with anH₂ oxidation reaction catalyst (not shown). Electrolyte solution 208(FIG. 2) is flowed into electrolyte flow field 404, while oxidant 236,here, protons, are formed in the oxidant flow field 408. FIG. 4Billustrates the operation of example oxidation cell 400, with block 428representing the cathode side of the oxidation cell that receives H₂O236, block 432 representing the anode side of the oxidation cell thatreceives electrolyte solution 208P, and block 436 representingproton-exchange membrane 412 that allows hydrogen protons to pass fromthe cathode side to the anode side, with the flow of electrons followingsuit. FIG. 4B also illustrates the oxidation of vanadium-basedelectrolyte solution 208P and the reduction of the protons into H₂ gas,which is removed by H₂O 236. Those skilled in the art will readilyunderstand the construction of reduction cell 400 illustrated in FIG. 4Ais merely an example and that other constructions may be used asdesired.

Inputs for example process 200 of FIG. 2 are:

-   -   Materials:        -   Generally, a solvent 244 for dissolving metal oxides 216,            here V₂O₅ powder. For the vanadium compound of the            illustrated example, this solvent is composed of one or more            strong acids, such as sulfuric acid and/or hydrochloric            acid. In other embodiments, other solvents, such as            hydrobromic acid, and chloric acid, among others, can be            used.        -   If the forgoing solvent is not a polar solvent, a polar            solvent 248. For the illustrated vanadium compound, water is            used.        -   An electrolyte, such as electrolyte 216. In the present            example, the electrolyte is vanadium pentoxide (V₂O₅)            powder. In other embodiments one or more vanadium oxides may            be used, such as vanadium (III) oxide (V₂O₃) alone or in            combination with vanadium (V) oxide (V₂O₅). In other            embodiments, other electrolytes, such as iron-chrome flow            battery, and all uranium flow battery, among others, can be            used.        -   A proton-donating reductant, such as reductant 212. In the            present example, H₂ gas is used. In other embodiments,            another reductant, such as hydrogen, water, formic acid,            ethylene glycol, H₂O₂, among others, can be used.        -   A proton-consuming oxidant, such as oxidant 232. In this            example, protons were reduced to H₂ gas. An alternative            could be to use oxygen or Air, where O₂ is reduced to H₂O.    -   Electricity:        -   Electricity (not illustrated) provides the energy input into            both the reduction cell and oxidation cell.

The final purified and adjusted electrolyte solution 208P+A in thepresent example (i.e., a vanadium-based electrolyte solutionvalence-adjusted to 3.5 valence) may contain:

-   -   electrolyte with a supporting acid solution (generally sulfuric        acid and/or hydrochloric acid);    -   a balance of vanadium (III) and (IV) ions in solution in        generally a 1:1 ratio; and    -   additional additives for thermal stabilization.

In this example, reduction/purification sub-process 200(1) involves theelectrochemical reduction of vanadium electrolyte solution 208. Thechemical/electrochemical reactions for this exemplaryreduction/purification sub-process 200(1) are shown below in Table I.

TABLE I Eqn. 1: V₂O_(5(s)) + 2H⁺ 

 2VO₂ ⁺ + H₂O V₂O₅ dissolution Eqn. 2: 2VO₂ ⁺ + 4V²⁺ + 8H⁺ 

  Chemical Reduction of 6V³⁺ + 4 H₂O V(V) reduction by V(II) Eqn. 3:6V³⁺ + 6e− 

 6V²⁺ Electrochemical reduction of V(III) ion in reduction cell −cathode reaction Eqn. 4: 3H₂ 

 6H⁺ + 6e− Electrochemical oxidation of reductant in reduction cell (H₂as example reductant) − anode reaction Eqn. 5: V₂O_(5(s)) + 4H⁺ + 3H₂ 

  Net reaction for reduction/ 2V²⁺ + 5H₂O purification sub-process

One of the main challenges of using V₂O₅ powder (electrolyte 216) is itslimited solubility in strong acid solutions. Instead of making avanadium (V) electrolyte solution, this exemplary process utilizes areduced electrolyte solution of predominantly vanadium (II) ions to bothdissolve and reduce the V₂O₅ powder. V₂O₅ powder, water, and acid areslowly added to a well-mixed tank (tank 252 in FIG. 2) containingelectrolyte solution 208 of predominately vanadium (II) ions. Thevalence of predominantly vanadium (II) solution 208 in tank 252 ismaintained via reduction cell 204. When the V₂O₅ powder (metal oxide216) is dissolved (Table I, Eqn. 1) and reduced (Table I, Eqn. 2) inpredominantly vanadium (II) solution 208, other vanadium ions areoxidized (Table I, Eqn. 2). This oxidation of vanadium ions is balancedusing reduction cell 204, which continuously reduces electrolytesolution 208 to maintain a constant valence. Measurements from a set ofsensors (not shown) that measure the vanadium (II) and (III)concentration and the total volume may be used to control the feed ratesof V₂O₅ (metal oxide 216), acid (solvent 244), and water (polar solvent248) into Tank 252, the electrical current into reduction cell 204, andthe extraction rate of approximately 2.0 valence electrolyte 208(2.0)from the outlet 2040 of the reduction cell to a tank 256 of oxidationsub-process 200(2). Examples of sensors for measuring the vanadium (II)and (III) concentration include commercial off-the-shelf optical sensorsand electrochemical cells having reference electrodes, among others.

The vanadium (II) ions in tank 252 serve two purposes. First, they aidin the rapid dissolution and reduction of V₂O₅ powder (metal oxide 216),as described previously. Second, the vanadium (II) ions provide areducing atmosphere to reduce many of the most deleterious impurities(e.g., impurities 220) to their solid, neutral states. When in theirsolid states, the impurities are filtered out using mechanical means,again, here a series of course filter 224 and fine filter 228(1) placedupstream of reduction cell 204. In the example shown, coarse filter 224,for example, an activated carbon filter, filters relatively largerprecipitated solids, and fine filters 228(1) and 228(2), for example,PTFE hydrophilic filters, filter relatively smaller precipitatedparticles. As noted above, in a VRFB context, the level of filtrationcan be dependent on the pore size of the negative electrode(s) used inthe target VRFB.

In this example impurities 220 that precipitate out of reducedelectrolyte solution 208 can include, but are not limited to, As and Gemetal precipitates. Filters 224, 228(1), and 228(2) before reductioncell 204 serve two functions: they 1) remove impurities 220 fromelectrolyte solution 208 and 2) protect the reduction cell from theprecipitated impurities. The second function is successfully achieved ifany of filters 224, 228(1), and 228(2) has a smaller effective pore sizethan the carbon-paper electrode of the reduction cell 204, such ascarbon paper electrode 316 of FIG. 3A. The system of filters 224,228(1), and 228(2) and reduction cell 204 shown in FIG. 2 allows for thecontinuous operation of the reduction cell. Methods that useelectroplating/electrowinning would require frequent chemical cleaningof the cell or electrode replacement, temporarily shutting downoperation.

Reduction cell 204 operates by reducing vanadium (III) ions at cathode204(C) (Table I, Eqn. 3) and oxidizing reductant 212 at anode 204(A). Asnoted above, in this example H₂ gas is used as reductant 212 (Table I,Eqn. 4), but as also noted above, other chemical reductants could beused, such as water, formic acid, or ethylene glycol, among others. Inthis example, reductant 212, here H₂ gas, is provided both from an H₂source 260 and an oxidation cell 232 of oxidation sub-process 200(2),which produces H₂ gas. An example of construction of reduction cell 204and the appropriate half-cell reactions are shown in FIGS. 3A and 3B,respectively. Downstream of outlet 2040 (FIG. 2) of reduction cell 204,a portion of purified electrolyte solution 208P is returned to tank 252to aid in the dissolution and reduction of V₂O₅ powder (metal oxide216), and a portion is moved to oxidation sub-process 200(2).

Oxidation sub-process 200(2) involves oxidizing purified electrolytesolution 208P. The chemical/electrochemical reactions for this exemplaryprocess are shown below in Table II.

TABLE II Eqn. 6: H⁺ + e− 

 ½ H₂ Hydrogen gas generation (cathode reaction) Eqn. 7: V³⁺ + H₂O 

 VO²⁺ + 2H⁺ + e⁻ Vanadium electrolyte oxidation (anode reaction) Eqn. 8:V³⁺ + H₂O 

 ½H₂ + VO²⁺ + H⁺ Net reaction for oxidation cell process

Purified electrolyte solution 208P of average valence that is below thecritical precipitation valence (i.e. below 2.9) transferred fromsub-process 200(1) into tank 256. Purified electrolyte solution 208P intank 256 is kept just below the final desired valence (generally 3.5 inthis example). Purified electrolyte solution 208P from tank 252 ispumped into oxidation cell 232, which oxidizes the purified electrolytesolution to the desired final valence to make valence-adjustedelectrolyte solution 208P+A. A portion of the output of oxidation cell232 is returned to tank 256 to maintain a constant valence, and, in thepresent example, the remainder is transferred into a VRFB, as indicatedby box 240.

In this example, electrochemical oxidation cell 232 oxidizes purifiedelectrolyte solution 208P and reduces protons (i.e., produces H₂ gas).Specifically, oxidation cell 232 oxidizes vanadium (III) ions at theanode 232A via the half-reaction shown in Table II, Equation 7, andreduces protons to form H₂ gas at the cathode 232C, as described by thehalf-reaction shown in Table II, Equation 6. The net reaction forexample oxidation cell 232 is given in Table II, Equation 8. Inoxidation cell 232, water is circulated on the H₂-producing side (i.e.,cathode 232C), as it helps wash away any vanadium ions that migrate overthe membrane 232M to the cathode.

As described above, an alternative process could produce purifiedvalence-adjusted electrolyte solution 208P+A at any desired valence. Forexample, oxidation cell 232 could oxidize a first batch of purifiedvalence-adjusted electrolyte solution 208P+A to 2.5 valence for thenegative electrolyte solution of a VRFB and a second batch of thepurified valence-adjusted electrolyte solution to 4.5 valence for thepositive electrolyte solution of the VRFB. Transferring these twoseparate solutions into, respectively, the catholyte tank and theanolyte tank of a VRFB would eliminate the need for the formationcharging process required in commissioning a new battery system.

In another embodiment, shown in FIGS. 12A and 12B, each of one or moreelectrochemical cells, here electrochemical cell 1200 having a negativeside 1200N (FIG. 12A) and a positive side 1200P (FIG. 12B), accepts anelectrolyte solution, such as a vanadium-based electrolyte solution, onboth the positive and negative sides of the cell. This eliminates theneed for a separate reductant for the reduction/purification sub-processand a separate oxidant for the oxidation sub-process. FIG. 13illustrates and example electrochemical cell 1300 that can be used aselectrochemical cell 1200 of FIGS. 12A and 12B. Referring to FIG. 13,electrochemical cell 1300 may have a symmetric design comprised ofnegative and positive flow fields 1304N, and 1304P, respectively,negative and positive carbon paper electrodes 1308N and 1308P,respectively, and a proton-exchange membrane 1312 between the negativeand positive sides of the cell.

In one example for purifying a vanadium-based electrolyte solution, thestarting impure electrolyte solution 1204 (FIG. 12A) may be made ofroughly equal amounts vanadium (III) and vanadium (IV) and contains atleast one impurity to be removed by the process. Impure electrolytesolution 1204 could be made, for example, using either of the methodsdescribed in the Background section above (i.e., either Method 1 orMethod 2). In the process illustrated in FIGS. 12A and 12B, the negativeelectrode 1200N of each electrochemical cell 1200 reduces electrolytesolution 1204 to below a critical impurity precipitation valence (see,e.g., zone 104 of FIG. 1), while the positive electrode 1200P of eachelectrochemical cell oxidizes vanadium (III) to vanadium (IV). Upstreamof the negative electrode(s) 1200N (FIG. 12A), one or more filters,represented schematically at 1212 of FIG. 12A and which can be the sameas or similar to filters 224, 228(1), and 228(2) of FIG. 2, captureprecipitated impurities in the negative-side electrolyte solution 1204Nprior to them entering negative electrode 1200N.

After passing through filter(s) 1212, a portion of this now-purifiedelectrolyte 1204P solution can, for example, be transferred to apositive electrolyte tank 1216 (FIG. 12B). This transfer processcounteracts the electrochemical oxidation of the fluid in thepositive-side electrolyte loop 1220 (FIG. 12B) and maintains anapproximately 3.5 valence. Similarly, starting electrolyte solution 1200(FIG. 12A) of lower-purity is slowly transferred to a negative-sideelectrolyte tank 1224. This transfer chemically counteracts theelectrochemical reduction and maintains a valence at or below thecritical precipitation valence. The solution in positive-sideelectrolyte loop 1228 (FIG. 12B) (stored in positive-side tank 1216) isfree of the key contaminants that would precipitate in a VRFB system.The positive-side purified and oxidized electrolyte solution 1204P+0 isfree deleterious impurities and is transferred to a holding tank 1232,where it may be stored prior to use, for example, in a VRFB system.While this embodiment adds additional material costs for the electrolytedue to the chemical reducing agent or the reduced vanadium oxide, itsprimary function is to remove key impurities from the electrolyte. Whenindividual electrochemical cells use the same basic electrolyte solutionon both their positive and negative sides, these cells may beconveniently called “electrolyte-only electrochemical cells.”

It is noted that on the reduction/purification side (FIG. 12A), thereduction/purification process 1236 may be controlled by an appropriatecontroller 1240 that controls the reduction/purification sub-process.Like controller 264 of FIG. 2, controller 1240 of FIG. 12A may includeany suitable hardware, such as programmable logic controller, generalpurpose computer, application-specific integrated circuit, or any otherhardware device(s) capable of executing a suitable control algorithm.Many types of hardware suitable for controller 1240 are well known inthe art. In one example, controller 1240 is configured, via software orotherwise, to control the valence of the electrolyte solution flowingout of negative side 1200N of reduction cell 1200, here electrolytesolution 1204P by controlling the flow of impure electrolyte solution1204 from negative-side electrolyte tank 1224 into the reduction cell.In this example, inputs to the control algorithm include user settings,such as the electrical current within reduction cell 1200, and a valencemeasurement taken of the electrolyte 1208 in the tank 1224 by a suitablevalence sensor. For example, with a fixed cell current, controller 1240determines whether or not the measured valence is below a target value(e.g., the precipitation valence) and outputs one or more controlsignals that control one or more flow-control devices (not shown) thatcontrol the flow of impure electrolyte solution 1204 into reduction cell1200. Such flow-control devices may be, for example, one or more pumps,one or more valves, or one or more devices that changes the flow impurefeedstock into the negative-side electrolyte tank 1224, or anycombination of these, among others. In other embodiments, controller1240 can be configured to control both cell current and impureelectrolyte solution flow so as to control the valence of theelectrolyte solution (here, solution 1208). Those skilled in the artwill readily understand how to create a suitable algorithm for thecontrol scheme selected for controller 1240 based on this disclosure andconsidering the type of hardware used.

FIG. 14 illustrates the operation of example electrochemical cell 1200(FIGS. 12A and 12B), with block 1400 representing the cathode side ofthe electrochemical cell that receives electrolyte solution 1204 fromnegative side electrolyte tank 1224 (FIG. 12A), block 1404 representingthe anode side of the electrochemical cell that receives electrolytesolution 1204 (mix) from positive-side tank 1216 (FIG. 12B), and block1408 representing proton-exchange membrane 1312 (FIG. 13) that allowshydrogen protons to pass from the cathode side to the anode side. Thoseskilled in the art will readily understand the construction ofelectrochemical cell 1200 illustrated in FIGS. 12A and 12B is merely anexample and that other constructions may be used as desired.

Lab-Scale Experimental Results

Overview and Summary of Lab Scale Results

A purification process in accordance with aspects of the presentdisclosure was demonstrated on the lab scale. Following is an overviewof that process.

-   -   A desirable industrial-scale system is a system that performs        continuous electrolyte formation/purification. However, for        proof of concept at the lab scale, a batch process was used.    -   Reduction of the 3.5 valence initial electrolyte solution was        done using a H₂/VRB hybrid electrochemical cell 500 (FIG. 5)        identical to the cell illustrated in FIG. 3A.    -   Oxidation of the purified electrolyte solution was performed in        a hybrid electrochemical cell 800 (FIG. 8) identical to the cell        illustrated in FIG. 4A.

Reduction/Purification Sub-Process Demonstration at Lab-Scale

Two samples of an initial electrolyte solution having a starting valenceof 3.5, i.e., Batch 1 and Batch 2, were subjected to a version of thereduction/purification process described above. Each sample was 3 litersin volume and was reduced to a solution of 2.0 valence. The vanadiumcontent of each sample was between 1.4 mol/liter and 1.65 mol/liter. Thesame reduction cell 500 (FIG. 5) (no electrode replacement or membranereplacement) was used for both samples. The construction of reductioncell 500 is identical in construction and operation to reduction cell300 as illustrated in FIGS. 3A and 3B, respectively. The active area ofreduction cell 500 (FIG. 5) was 23 cm².

FIG. 5 illustrates the lab-scale reduction/purification sub-process usedin the testing of Batches 1 and 2. A hydrogen cylinder 504 providedhydrogen to an H₂/VRB cell 508 on the positive electrode 508P. The dryhydrogen was humidified by passing it through deionized water (DI) in abubbler 512. A flow regulator 516 controlled the flow of hydrogen intoH₂/VRB cell 500. A VRB electrolyte 520 was stored in a plastic tank 524.VRB electrolyte 520 in tank 524 was recirculated through H₂/VRB cell 508on the negative electrode 508N and passed through two filters 528(1) and528(2) prior to entering the cell. Recirculation was performed by aperistaltic pump (not shown). A DC current (not illustrated) was appliedto H₂/VRB cell 508, which oxidized the hydrogen and reduced the vanadiumin VRB electrolyte 520. Filters 528(1) and 528(2) upstream of H₂/VRBcell 508 consisted, respectively, of a coarse filter (activated carbon)and a fine filter (hydrophilic PTFE filter with 0.5-micron pore size).Cell voltages in H₂/VRB cell 500 are shown in FIG. 6A for Batch 1 and inFIG. 7A for Batch 2, and cell pressures are shown in FIG. 6B for Batch 1and in FIG. 7B for Batch 2. Open-circuit voltages reported in FIGS. 6Aand 7A were measured by periodically removing DC current from the cell.The reduction/purification sub-process appears to have been successfulin both instances, demonstrating two key concepts, namely:

-   -   The pressure (FIGS. 6B and 7B) measured upstream of filters        528(1) and 528(2) (FIG. 5) increased as the process continued.        This pressure rise indicates that filters 528(1) and 528(2) are        successfully capturing impurities that are precipitating.    -   The pressure (FIGS. 6B and 7B) just upstream of reduction cell        508 (FIG. 5) does not increase during the sub-process,        indicating that filters 528(1) and 528(2) protect the reduction        cell from clogging because of precipitated impurities.

Oxidation Sub-Process Demonstration at Lab-Scale

Batch 1 of the purified electrolyte solution from thereduction/purification sub-process was oxidized to a 3.5 valence usingan H₂/VRB cell 800 (FIG. 8), which was identical in construction andoperation to the construction and operation of oxidation cell 400 shownin FIGS. 4A and 4B, respectively. The batch-type oxidation sub-processperformed at the lab scale is illustrated in FIG. 8. DI water was storedin a plastic tank 804, where it is recirculated through H₂/VRB cell 800on the negative electrode. The flow of DI water helped remove anyvanadium that migrated over to the negative electrode 800N. VRBelectrolyte 808 was stored in plastic tank 812 and was recirculatedthrough H₂/VRB cell 800 on the positive electrode 800P of the cell. A DCcurrent (not illustrated) was applied to H₂/VRB cell 800, which reducedprotons to hydrogen gas and oxidized the vanadium in VRB electrolyte808. The voltage and current densities within H₂/VRB cell 800 are shown,respectively, in FIGS. 9A and 9B. The active area of H₂/VRB cell 800(FIG. 8) was 23 cm².

Verification of Electrolyte Purification at Lab-Scale

Batch 1 was tested in a sub-scale VRB system to verify that the keyimpurities had been removed to an acceptable level in the electrolyte.The performance of the electrolyte before and after the purificationprocess is shown in FIG. 10, which compares the negative electrodepressures during operation before and after the lab-scale purificationprocess. Full cell operation metrics for the purified Batch 1electrolyte (voltages, cell resistances, cycle performance, andpressures) are shown in FIGS. 11A to 11C. In FIG. 11B, “CE” stands forCoulombic efficiency, “EE” stands for energy efficiency, and “Util”stands for utilization of vanadium. Data represented in FIG. 11C are fornegative-side pressure and positive-side pressure with a lab-scale fullVRB cell with respective vanadium-based electrolyte solutions flowing oncorresponding respective sides of the cell.

An overview of the system parameters and specifications is given belowand illustrated in FIGS. 11A to 11C:

-   -   Cycling parameters:        -   Open-circuit voltage at end of discharge=1.28 V        -   Open-circuit voltage at end of charge=1.52 V        -   Max charge cell voltage=1.6 V    -   System Specifications:        -   1.46 mol/liter V        -   3.50 valence        -   2.8 liter total system volume (positive and negative            electrolytes)        -   2× Carbon paper electrodes        -   Proton exchange membrane        -   11 cm² active area

The foregoing has been a detailed description of illustrativeembodiments of the invention. It is noted that in the presentspecification and claims appended hereto, conjunctive language such asis used in the phrases “at least one of X, Y and Z” and “one or more ofX, Y, and Z,” unless specifically stated or indicated otherwise, shallbe taken to mean that each item in the conjunctive list can be presentin any number exclusive of every other item in the list or in any numberin combination with any or all other item(s) in the conjunctive list,each of which may also be present in any number. Applying this generalrule, the conjunctive phrases in the foregoing examples in which theconjunctive list consists of X, Y, and Z shall each encompass: one ormore of X; one or more of Y; one or more of Z; one or more of X and oneor more of Y; one or more of Y and one or more of Z; one or more of Xand one or more of Z; and one or more of X, one or more of Y and one ormore of Z.

Various modifications and additions can be made without departing fromthe spirit and scope of this invention. Features of each of the variousembodiments described above may be combined with features of otherdescribed embodiments as appropriate in order to provide a multiplicityof feature combinations in associated new embodiments. Furthermore,while the foregoing describes a number of separate embodiments, what hasbeen described herein is merely illustrative of the application of theprinciples of the present invention. Additionally, although particularmethods herein may be illustrated and/or described as being performed ina specific order, the ordering is highly variable within ordinary skillto achieve aspects of the present disclosure. Accordingly, thisdescription is meant to be taken only by way of example, and not tootherwise limit the scope of this invention.

Exemplary embodiments have been disclosed above and illustrated in theaccompanying drawings. It will be understood by those skilled in the artthat various changes, omissions and additions may be made to that whichis specifically disclosed herein without departing from the spirit andscope of the present invention.

What is claimed is:
 1. A method of making a purified electrolytesolution for use in a vanadium redox flow battery, the methodcomprising: providing a vanadium-based electrolyte solution having aninitial average valence and containing at least onenon-vanadium-containing impurity having a precipitation valence below3.0 and that precipitates out of the vanadium-based electrolyte solutionwhen the valence of the electrolyte solution is at or below theprecipitation valence; reducing the electrolyte solution from theinitial average valence to a valence below 3.0 so as to cause the atleast one non-vanadium-containing impurity to precipitate out of theelectrolyte solution as a solid precipitate; and while the valance ofthe electrolyte solution is below 3.0, mechanically separating the solidprecipitate out of the electrolyte solution using at least one of 1) aflow-through porous filter and 2) a hydrocyclone so as to make thepurified electrolyte solution.
 2. The method according to claim 1,wherein the reducing of the vanadium-based electrolyte solution to anaverage valence of below 3.0 includes reducing the vanadium-basedelectrolyte solution to an average valence of less than 2.5.
 3. Themethod according to claim 1, wherein the reducing of the vanadium-basedelectrolyte solution to an average valence of below 3.0 includesreducing the vanadium-based electrolyte solution to a valence ofsubstantially 2.0.
 4. The method according to claim 1, wherein thereducing of the vanadium-based electrolyte solution is performed using ahybrid electrochemical cell.
 5. The method according to claim 4, whereinthe reducing of the vanadium-based electrolyte solution utilizeshydrogen gas as a reductant.
 6. The method according to claim 1, furthercomprising oxidizing the purified electrolyte solution to the desiredvalence prior to installing the purified electrolyte solution into thevanadium redox flow battery.
 7. The method according to claim 6, whereinthe desired valence is substantially 3.5.
 8. The method according toclaim 6, wherein the desired valence is substantially 4.5.
 9. The methodaccording to claim 6, wherein the desired valence is substantially 2.5.10. The method according to claim 1, further comprising, prior toinstalling the purified electrolyte solution into the vanadium redoxflow battery, oxidizing at least a portion of the purified electrolytesolution to each of two valences so as to create a plurality of purifiedvalence-adjusted electrolyte solutions of differing valence.
 11. Themethod according to claim 10, wherein oxidizing at least a portion ofthe purified electrolyte solution includes oxidizing a first portion ofthe purified electrolyte solution to an average valence of greater than3.5 and oxidizing a second portion of the electrolyte solution to anaverage valence of less than 3.5.
 12. The method according to claim 11,wherein the oxidizing of a first portion of the purified electrolytesolution to an average valence of greater than 3.5 includes oxidizingthe first portion to an average valence of about 4.5 and oxidizing asecond portion of the purified electrolyte solution to an averagevalence of less than 3.5 includes oxidizing the second portion to anaverage valence of about 2.5.
 13. The method according to claim 10,wherein the oxidizing of at least a portion of the purified electrolytesolution is performed using a hybrid electrochemical reduction cell. 14.The method according to claim 13, wherein the oxidizing of at least aportion of the purified electrolyte solution utilizes the formation ofhydrogen gas from protons.
 15. The method according to claim 14, whereinthe reducing of the purified electrolyte solution is performed using ahybrid electrochemical reduction cell.
 16. The method according to claim15, wherein the reducing of the purified electrolyte solution utilizeshydrogen gas output from the hybrid electrochemical oxidation cell. 17.The method according to claim 10, wherein the reducing of thevanadium-based electrolyte solution and the oxidizing of at least aportion of the purified electrolyte solution are performed using anelectrolyte-only electrochemical cell having a reduction side and anoxidation side with the vanadium-based electrolyte solution on thereduction side and the purified electrolyte solution on the oxidationside.
 18. The method according to claim 1, wherein all vanadium in thevanadium-based electrolyte solution comes substantially only from mixingV₂O₅ with at least one strong acid.
 19. The method according to claim18, wherein all the vanadium in the vanadium based electrolyte solutioncomes substantially from V₂O₅ and V₂O₃, each mixed with at least onestrong acid.
 20. The method according to claim 1, wherein thevanadium-based electrolyte solution has a lowest possible valence andthe reducing of the vanadium-based electrolyte solution includesreducing the vanadium-based electrolyte solution to about the lowestvalence.